Figure 1.

Three scalpellid species representing different sexual systems. (a,b) Scalpellum scalpellum. Androdioecious species attached to hydroids. (a) A solitary hermaphrodite (note the penis; upper inset) that depends on dwarf males (lower inset) for reproduction. (b) A small group that can reproduce by cross-fertilization even without dwarf males. (c) Trianguloscalpellum regium. This deep-sea dioecious species depends entirely on the presence of dwarf males for fertilization. The receptacle inside the scutal edge (inset) contains numerous males (asterisk indicates a metamorphosing cyprid). (d) Pollicipes pollicipes. This hermaphroditic species inhabits the upper rocky intertidal zone in dense populations, which ensures cross-fertilization. Scale bar, 1 cm.